The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to an image forming apparatus of the type including a revolver type developing unit adjoining an image carrier and having a plurality of developing sections arranged around a center shaft thereof.
An image forming apparatus of the type described and capable of sequentially locating a plurality of developing sections at a preselected developing position is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-6467. The prerequisite with this type of apparatus is that the developing sections be sequentially brought to and accurately positioned at the developing position. To meet this requirement, a developing unit including the developing sections may be driven by a stepping motor. A stepping motor can have its rotation angle delicately control led on the basis of a reference position and the number of pulses to be applied to the motor. With a stepping motor, therefore, it is possible to accurately position any one of the developing sections at the developing position without resorting to any mechanical positioning scheme.
The above mechanical positioning scheme may be implemented by notches formed in the circumferential edge of the rear end wall of the developing unit and respectively assigned to the developing sections, and a roller rotatably mounted on the apparatus body. The roller engages with one of the notches when the subunit corresponding to the notch is brought to the developing position. For accurate positioning, the developing unit may be rotated by an angle greater than an angle necessary for one notch to meet the roller and then rotated in the reverse direction. This, however, increases the angular distance over which the developing unit should move, and therefore increases the positioning time. While a solenoid may be applied to the mechanical positioning scheme, the solenoid needs an extra start-up time.
Therefore, a stepping motor used to position the developing unit alone is advantageous over the mechanical positioning scheme from the high speed operation stand point also.
Toner cartridges each storing toner of particular color are also mounted to the developing unit, and each feeds the respective toner to an associated one of the developing sections. When a toner near-end condition is sensed in any one of the developing sections and when the associated toner cartridge is empty, the cartridge is replaced with a full toner cartridge.
In the apparatus of the type positioning the developing unit only with a stepping motor, when the feed of current to the motor is stopped while image formation is not under way, the developing unit originally freely rotatable in the apparatus is made entirely free and can be rotated even by hand. This brings about a problem that when the operator inserts a new toner cartridge into a particular developing section in the developing unit at the time of replacement, the developing unit rotates and prevents the outlet of the cartridge from aligning with the above developing section. As a result, toner or the cartridge itself is apt to drop.
In light of the above, Japanese Patent Laid-Open Publication No. 10-260581, for example, teaches an image forming apparatus including a stepping motor for rotating a developing unit having a plurality of developing sections and a plurality of replaceable developer containers. At the time of replacement of any one of the developer containers, the phase of the stepping motor is continuously excited such that the motor exerts on the developing unit a force preventing the unit from being rotated by an unexpected extraneous force. By preventing the developing unit from rotated during replacement, it is possible for the operator to surely replace a desired developer container.
However, the problem with the above apparatus taught in Laid-Open Publication No. 10-260581 is that throughout the replacement of the developer container performed by the operator, a current is continuously fed to the stepping motor which originally should drive the developing unit. As a result, when a current or a voltage driving the motor sharply changes due to, e.g., an accidental change in a signal for commanding the rotation of the motor, the developing device in the form of a revolver is apt to suddenly start rotating.
To promote the high speed operation of the above apparatus, it is necessary to load the developing device with a great amount of developer and a great amount of toner, increasing the overall weight and size of the device and therefore the moment of inertia. It follows that a stepping motor free from the loss of synchronism, or step-out, must be used. However, a stepping motor obviating defective images ascribable to step-out despite heavy loads and high speed rotation is not feasible for general purpose applications and is therefore extremely expensive. This, coupled with the fact that the stepping motor needs an AC power source, complicates the construction and makes it difficult to mount the motor on a copier, printer or similar office equipment. It is therefore difficult to enhance high speed operation. While the voltage or the current for driving the motor and therefore the output torque of the motor may be increased, this kind of approach aggravates power consumption.
It has been customary with the image forming apparatus to transmit the output torque of the stepping motor mounted on the apparatus body to the center shaft of the developing unit via a plurality of intermeshing gears. On the other hand, during image formation, the developing unit is rotated only in one direction and, in principle, not rotatable in the other direction. However, because play ascribable backlash exists between the intermeshing gears, the developing unit is likely to slightly rotate in the reverse direction, resulting in defective images.
Technologies relating to the present invention are also taught in, e.g., Japanese Patent Laid-Open Publication Nos. 10-142941, 60-208779, 61-103175, 61-151564, 10-114180, 61-077873, 62-251772, 63-178262, and 5-94086.